In recent years, to reduce a burden imposed on a driver, a device is developed which provides various kinds of driving assistance to the driver. For example, a lane keeping device is known in which a pair of white lines are detected from an image obtained by capturing the vehicle's front side, and steering assist torque is provided to a steering mechanism such that the vehicle travels along the center (target locus) of the lane constituting the pair of white lines (see Patent Citation 1). In this lane keeping device, upon steering control, feedback control is performed, how much the actual position of the vehicle is shifted from the target locus (that is, an error) is detected, the steering assist torque is calculated so as to reduce the error, and the steering assist torque is generated.    [Patent Citation 1] Japanese Unexamined Patent Application Publication No. 11-321690    [Patent Citation 2] Japanese Unexamined Patent Application Publication No. 2007-38750    [Patent Citation 3] Japanese Unexamined Patent Application Publication No. 2002-12159    [Patent Citation 4] Japanese Unexamined Patent Application Publication No. 2002-120744    [Patent Citation 5] Japanese Unexamined Patent Application Publication No. 2005-225430